73 NAVSTAR GPS AND OTHER SATELLITE NAVIGATION SYSTEMS

290. OVERVIEW OF THE NAVSTAR GLOBAL POSITIONING SYSTEM (GPS)

In an attempt to achieve an even better satellite navigation system, the US armed forces, under the direction of a joint services programme, have been involved in the development of a completely new satellite navigation system known as GPS.  Currently the GPS is in its final phase of development and there are now a total of 24 space vehicles (SV’s) in orbit (21 active plus 3 spares).  The GPS will replace the NNSS with which it is not compatible.

The final total of 21active SV’s are launched into seven 12-hour orbital planes each containing three satellites.  The altitude of each SV is stabilised within 1 metre by the action of four reaction wheels. Hydrazine thrusters enable orbital position to be maintained by providing precision realignment of the SV as required.

The 12 hour orbital period is in sidereal time, not solar time, so that each SV noon position will shift each day because the sidereal day is 4 minutes shorter than the solar day. Each SV in any one orbit is affected equally hence satellite spacing per orbit remains constant. The seven orbits are at an altitude of approximately 1,650 km and each orbit has an inclination of 55° (see fig. 73-1).

Orbital altitude and inclination have been chosen so that SV telemetry can be controlled by Earth stations situated in US territory. This orbital configuration encompassing 21 SV’s ensures that at least four SV’s with an elevation greater than 9°.5 will be in view of the receiving antenna at any point on the Earth’s surface at any one time (9°.5 has been found to be the minimum acceptable elevation for a satellite pass with a simple antenna system).

Navstar GPS is controlled from Vandenburg US Air Force Base in California. There are four monitor stations situated at Vandenburg, Hawaii, Guam and Elmendorf (Alaska). Tracking data are fed to the Control Station where future orbital data are predicted and transmitted to individual SV’s. The SV returns the data in its computer memory, re-transmitting them as required to provide a Data Frame for the receiving stations.

Navstar GPS is a multi-modal and multi-user, passive navigational system in which users simply receive radiated signals. This navigational aid provides highly accurate positional and velocity information in three dimensions on a global basis, continuously, unaffected by weather, to an unlimited number of properly equipped users.  The system’s signals operate in the lower part of the microwave band and are thus relatively unaffected by atmospheric and ionospheric conditions.

The system concept is based upon the accurate and continuous knowledge of the spatial position of each satellite in the system with respect to time, and the time of transmission of the signal from the satellite to the user. The user system automatically selects appropriate signals from each of three or four satellites ‘in view’ with respect to optimum satellite-to-user geometry.

It then solves the three time-of-arrival difference quantities to obtain distance between user and satellites.  This information establishes the user position with reference to the satellite system.

Each satellite in the system transmits signals on two frequencies but only one of these, 1575.42 MHz is available to civilian users giving a potential accuracy in the order of a few metres, but this accuracy is degraded intentionally so that only a nominal 100 metres accuracy will be available.  This degradation is required to protect US national security interests.  The dual frequency system will only be available to military and certain other selected users with a potential accuracy of 5 – 10 metres.

A GPS position fix is achieved by the precise measurement of the distance between the satellite and the receiver at an instant of time.  The position of the satellites is known very accurately and transmitted in the data message from each SV, so these ranges will provide position lines, the intersection of which is the position of the receiver.  Measurement of the ranges will be on a continuous basis so that the position will be continually updated, the complex calculations necessary to translate the ranges into Lat. and Longitude read-outs being accomplished by the microcomputer built into the onboard receiver.

A GPS position fix can be in four dimensions (Lat., Longitude, altitude and velocity) for military users and for this three or four satellites would be needed. An Earth-bound station such as a yacht or ship, requires only two dimensional position fixing which can be achieved by receiving data from three satellites only. To measure the precise distance between the SV and the receiver requires highly accurate clocks on both vehicles.

The satellite clock is monitored from the ground and is corrected by atomic standard time (i.e., to one part in 1012).  It is accepted that the satellite clock, which is used to generate the transmission frequencies, is accurate and that the receiver clock may be in error. For this reason, range measurements are called false or ‘pseudo-ranges’ and most be corrected within the receiver microprocessors. The GPS receiver calculates the pseudo-range time taken from the transmission and compares it with a locally generated time in the microprocessor. Fig. 73-2(a) illustrates that the pseudo-ranges calculated for three satellites will not converge at a specific point unless the receiver clock error is corrected. The vessel’s position is solved with reference to charted co-ordinates as shown in fig. 73-2(b) with reference to three celestial ‘fixed’ points (the SV’s).

291. NAVSTAR GPS USER EQUIPMENT

Most of the major manufacturers have developed receivers for Navstar GPS, these being very similar to those previously available for NNSS.

We have seen that, although only single-channel GPS reception is made available to non-military users, accurate reliable navigation fixes can be obtained from a single channel, thus simplifying receiver design. To further reduce user equipment cost the receiver can sequentially track up to four SV’s (multi-channel equipment being intended to track four SV’s simultaneously). Fig. 73-3(a) illustrates a typical modern state-of-the-art GPS Receiver for small craft. The continual development of electronic equipment has ‘considerably reduced cost, size and power consumption so that very many small craft are now fitted with built-in or hand-held receivers. The one shown in fig. 73-3(b) is easy to use, has its own small batteries, is waterproof and can float.

Because no speed or heading input is required with Navstar receivers, operation is extremely simple, requiring little more from the operator than switching on and off. It will take a little while for the receiver to get up to date with the satellite characteristics but, once fully aligned, the receiver will give positions of a high order of accuracy on a continuous basis.  The forecast accuracy of 100 metres for commercial Navstar GPS makes the system more than adequate for both ocean and coastal navigation.

GPS receivers include a display and a small keyboard. Like the hyperbolic navigators (Decca and Loran-C), in addition to displaying the boat’s position in Lat. and Longitude they incorporate a computer capable of performing virtually all of navigational functions of a mariner almost simultaneously.  For example, waypoints can be entered as shown in fig. 73-4 and, when required, the display can show the bearing and distance of the next waypoint [as in fig. 73-3(b)].  They can also show the speed over the ground (SOG or SMG) and the course made good over the ground (COG or CMG) as shown in fig. 73-3(a).

292. CHART DATUM’S AND DATUM SHIFT

Lat. and Longitude are defined in terms of angles measured at the centre of the Earth.  There are problems in determining exactly where the centre of the Earth is. The geoid (see fig. 73-5) is an imaginary surface of the Earth which corresponds to the Mean Sea Level of the oceans. It is irregular in Shape and tends to rise under mountains and dip above certain basins.

Its irregularity precludes mathematical determination of Lat. and Longitude so a regular but fictional surface called the spheroid is used for the calculation, this being the closest fit to the geoidal section.

The Earth is an oblate spheroid, i.e., it is ‘flattened’ in the polar regions, so that the oblate spheroid is sometimes called an ‘ellipsoid’.  In different parts of the world reference datum’s for charting Lat. and Longitude have used different spheroids.

For example, the 1936 Ordnance Survey of Great Britain [OSGB 361 Datum used the Airy spheroid, the European Datum (1950) is based on the International spheroid of 1924, and the North American Datum (1927) is based on the Clarke spheroid of 1866.

In the days before satellite navigation this difference in the various datum’s used for charting had very little effect on the day to day navigation of ships. But the requirements of manned space flight and the development of worldwide satellite navigation systems demanded the establishment of an agreed worldwide spheroid which fitted the actual Shape of the whole Earth as closely as possible. Thus evolved the World Geodetic System 1972 (WGS 72).  The increasing world-wide use of this system shows up the discrepancies in the various datum’s previously used for charting, and this discrepancy, called the Datum Shift, is now noted near the title on charts not based on WGS 72 or WGS 84.

As technology has advanced, the WGS has been refined and developed. NAVSTAR GPS uses a newer WGS 84 Datum but, as far as the mariner is concerned, the difference between WGS 72 and WGS 84 is small. As an example, the Datum Shift off the E. coast of England is almost 150 metres, and in Southampton Water 130 metres between WGS 72 and OSGB 36.

Paradoxical as it may seem, the very accuracy of satellite navigation can make it dangerous. Many charts, particularly in the Pacific Ocean, were made before such pin-point fixing was possible, so that land masses and islands are not always charted in their correct positions, sometimes being a mile or more in error, while some atolls are regularly misplaced by up to 5 miles. Your very accurate GPS receiver will tell you exactly where you are on the surface of the globe, but when this is plotted on your chart you may think you are off a harbour entrance when in fact you are just about to run on to a reef.  Correction of charts for the Datum Shift is therefore essential.

293. SATELLITE NAVIGATION VERSUS ASTRO-NAVIGATION

Satellite navigation, and particularly GPS navigation, has made a big impact with ocean cruising yachtsmen.  Undoubtedly it is an invaluable back-up for occasions when closing land or when persistent cloud cover and rough seas make Astro-navigation difficult or perhaps impossible.

Possession of satellite navigation equipment is no excuse to stop the daily Astro-navigation routine, in which the small craft mariner must be well-versed in case the electronic system fails. GPS navigation provides a good check for sights and shows how accurate you can be with a sextant. It will help to provide the pinpoint accuracy required when closing land and it has opened up places like the Tuamotu Archipelago, where many fine mariners have lost their craft in the past owing to erratic currents and the inability to fix their positions with accuracy.

294. PROBLEMS ASSOCIATED WITH GPS IN EUROPE

As has been shown, the technical problems of GPS are few and those remaining are likely to be overcome in the near future.  However, there are other problems associated with the system, these being of a political nature.

Documents so far released show that GPS coverage over the United States is good. Away from the US the coverage expressed, as PDOP (Positional Dilution of Precision) deteriorates.  This happens, for instance, just N. of Scotland and around Gibraltar, and the deterioration could increase in the event of any satellite malfunction.

In addition, there is considerable anxiety in the international community about the inadequacy of the warning which GPS provides in the event of a system malfunction. It could be up to one hour before the user heard about a system failure.  In the US the Federal Aviation Authority is considering a multiplicity of independent monitors, providing near instant warning via geo-stationary satellites, but this will be of no benefit to European users.

A further problem is the possibility of access denial if this were in the interest of the United States, since the US has openly admitted that they would license operations or change the operating characteristics and signal formats (of GPS) during a dire national emergency”.

The UK has no influence or authority to affect the performance or the continuity of operation of GPS and, as such, the system cannot be totally relied upon as the sole or prime radio navigation system of the future for the British Isles, and this view is shared in Europe.

The marine satellite navigation situation needs to be carefully monitored but, until UK policy is settled, the GPS can only be recognised as useful, but not recommended to the marine community in the British Isles.  A sobering thought. Currently there is no charge levied upon the service. With pressure groups in the US demanding that the system be brought under civilian control, expensive licensing is a future possibility.

295. OTHER SATELLITE NAVIGATION SYSTEMS

Navstar GPS is only one of several new satellite navigation systems being developed or proposed, although it is the furthest advanced. In the former USSR a system very similar to Navstar is being developed on a similar time scale. This is called GLONASS and operates with 12 satellites providing two dimensional fixing. Russia already has the TSIKADA satellite navigation system in operation, this being very similar to the NNSS. Little use has been made of this system by other than Russian ships, and this situation may continue..

The European Space Agency (ESA), aware of the political implications posed by both the American and Russian systems, has proposed a purely commercial satellite navigation system called by the confusing name of NAVSAT using a constellation of 24 satellites. Another proposed system developed in W. Germany would use 20 satellites and is called GRANAS, but both Navsat and Granas would appear to have little chance of becoming operational.

An altogether different approach to satellite navigation is found in the American/European GEOSTAR which is a purely commercial system available for land, air and marine use and for which users are expected to pay.

This system is under development by the Geostar Corporation and comprises powerful land-based computers linked to two satellites which in effect are simply transponders, linking the control computers to the user’s transceiver. Range measurements from the returning signals via the two satellites are used to compute the user’s position in the central computer which then transmits the position back to the user.  This complex transaction takes place in a fraction of a second and the system is capable of handling a very large number of users simultaneously.  Accuracy is claimed to be in the region of one metre. Geostar is capable of many interesting developments if satisfactory commercial funding can be arranged.

At present there is no UK investment or controlling presence in any of the satellite navigation systems so far mentioned. Because of the technical supremacy of such systems over Earth-bound radiolocation systems it is important that the United Kingdom and Ireland should become formally involved and gain a position o influence in some satellite system so that it can be recommended with confidence to the marine community in the British Isles.

The most promising method of achieving this end would appear to be through the UK’s existing interest in INMARSAT (the International Maritime Satellite Organisation). Approximately 20% of the effective equity in Inmarsat is the share of the United Kingdom. The concept of an internationally controlled worldwide satellite navigation system is attractive, and Inmarsat, who control international marine satellite communications, is the logical body to run such a system. Proposals now being made are for Inmarsat to provide a marine navigation aid service from its geo-stationary satellites already in orbit using a ranging method from at least two space vehicles.